1. Field of the Invention
The present invention concerns the production of large composite elements requiring high dimensional precision.
More particularly, the invention is intended to produce elements formed of a support panel flanked on one or both sides of a carbon skin mounted by means of glueing, the aim of the invention being to obtain large light elements possessing high dimensional precision and extreme rigidity at a moderate cost.
2. Description of Background and Relevant Information
The sphere of application of the invention naturally concerns machines or structures intended to be placed in orbit around the Earth or launched into space, but also aeronautics in general, as well as maritime applications (boat structures or elements, such as hulls, for example).
The structural elements of the above-mentioned type, namely a panel flanked on one or both sides of a skin formed of one or several fiber laps preimpregnated with a thermosetting resin, each skin being fixed to the support panel with the aid of an adhesive resin, are generally produced in two stages: stratification of the skin(s) and hardening of the assembly adhesive. These two stages shall each involve a polymerization process, the two polymerization phases being carried out either consecutively by a traditional baking technique, or simultaneously, the polymerization of the preimpregnation resin of the fibers and the adhesive resin being effected by co-baking.
In this latter case, the support panel and the skin(s) is/are placed in a suitably shaped mold and all the above are placed in an autoclave so as to polymerize the resins under high pressure and at a high temperature.
The production by this technique of large elements poses several problems.
Firstly, the normal size of the autoclaves limits that of the molds able to be introduced into the latter and thus that of the elements able to be produced, unless of course special autoclaves are manufactured, this proving to be expensive.
Secondly, the temperatures implemented in this thermal polymerization are about between 160 and 180.degree. C. resulting in risks of internal stresses, microcracking and delamination, as well as the risks caused by the size of the elements of deformation by means of expansion, these risks requiring that resort may be made to using molds made of a material having an extremely low coefficient of heat expansion, such as materials made of invar or graphite which are expensive.
In addition, owing to the weights present and the heterogeneity of the element, a problem occurs concerning the progression of the thermal flow inside the unit to be polymerized, which renders it extremely difficult to control the thermal isotropy of the polymerization process. Finally, these thermal restraints result in working with slow temperature rises, which clearly adversely affect the cost of the operation.
Apart from its high cost, thermal co-baking thus results in obtaining less effective performances, especially as regards the precision of the shape characterized by the parameter normally known as the RMS (Root Mean Square).
Instead of a thermal co-baking or hot polymerization, it is possible to carry out an ionization co-polymerization whose principle is well-known and is known as cold polymerization owing to the induced temperature rises limited to about 80.degree. C.
There are certain advantages in this polymerization as regards its speed of implementation, its effectiveness and the weak thermal stresses imposed on the materials. It can be applied quite naturally to elements or objects possibly having large dimensions and does not lead to dimensional variations of the irradiated elements. However, the known applications of this technique, as in the European patent N.degree.0.165.118 granted in the name of the Applicant, mainly concern the simple irradiation of complete composite elements already embodied so as to harden the resins forming part of the composition of said elements. This irradiation is not used during the method for producing the element to its desired shapes and sizes, but subsequently and in normal or ambient pressure and temperature conditions.
Furthermore, polymerization by ionization does pose certain problems concerning the behavior of the resins.
In fact, from the start of this polymerization, the resin stiffens and does not pass through the virtually fluidity state it reaches in conventional thermal polymerization, which adversely affects a good diffusion, especially in the fiber intersite spaces.